1 University of West Hungary, Bajcsy-Zs. u. 4., H-9400 Sopron,
1abrahamj@fmk.nyme.hu
2 komansz@fmk.nyme.hu
3 borcsok.zoltan@fmk.nyme.hu@
Keywords: wood surface, colour change, glue, durability class
ABSTRACT
The bonding process is an important technology in the wood industry. The problems arise during bonding are in a wide palette range from the serious structural faults to the simple aesthetic problems. The aesthetic errors can also dramatically reduce the price of the product even if it has an ideal adhesive quality, so this is important to eliminate these errors. The discoloration of bonding (bonding plane) may caused the different bonding participants, materials in contact with one another, which can occur between:
1. adhesives and bonding materials 2. the glue and applicators
3. the bonding material and backhaul devices.
We studied the wood surface discolouration with the effects of different dispersion adhesives. Six adhesives were used, two of them were D3 (water resistance level), the others were D2. The most commonly used wood species in Hungary (oak, black locust, ash, beech, birch, Scots pine and spruce) were used.
The changes of the measured values in the CIELAB color coordinate system shows that each species responded differently depending on the type of adhesive used. In some cases, significant changes have caused the adhesives, especially the D3 type adhesive on wood surface containing a high amount of extracts.
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* This (research) was supported by the European Union and co-financed by the European Social Fund in frame of the project "Talentum - Development of the complex condition framework for nursing talented students at the University of West Hungary", project ID: TÁMOP 4.2.2.B-10/1-2010-0018
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INTRODUCTION
The surface of wood often suffers some discolouration during industrial gluing technology. Depending on the extent and the measure of this phenomenon, customers often claim about this problem. The user of the glues often claim the glue producer as the detected discolouration would be caused by the low quality, and/or changing quality of the glue. According to our experience in many cases the detected discolouration is a consequence of the inappropriate gluing conditions (e.g. applicators, temperatures) and the different wood species.
In the wood discoloration examination the measurements of color changes was applied in the past two decades, in particular the study of photodegradation (TANEDA ET AL.1989,TOLVAJ 1994/B,TOLVAJ AND FAIX
1995, CHANG AND CHANG 2001, AYADY ET AL. 2003,HANSMANN ET AL. 2006,OLTEAN ET AL.2008,2009,WANG AND REN 2008,SHARRATT ET AL. 2009,TOLVAJ AND MITSUI 2010).
During the discoloration caused by the reaction of the wood and the glue the extracted substances from the wood can react with components of the adhesive, which is observed soon after the bonding. Woods, containing other types of extract materials has other type of color change as the woods does not have extracts, or contains only small amounts of it. If the wood contains a high amount of extract material, the chemical processes of the color changes quickly take place. A number of studies made to study the effect of substances extracted from the wood on the photodegradation of wood (NÉMETH ET AL.1992, CHANG ET AL. 2010). The conclusions said that the resulting compounds will protect the wood from further degradation. At woods without extract materials, soon begins the photodegradation of the lignin with less but continous color change speed.
EXPERIMENTAL METHODS
The goal of the investigation was to determine how to change the appearance of the surface the interactions of dispersion adhesives and wood (most commonly used in Hungary). The changes of the surface were examined with color measurement. The color compositions of the woods were determined with Minolta CM-2600 spectrocolorimeter. The data are given in the CIE L* a* b* color coordinate system. The results pertain to D65 light source, measuring 8 mm in diameter surfaces, 10° observation. 6 different PVAc glues were tested, durability class: Type D2 (nr 1, 2, 5, 6), Type D3 (No.3, No.4). The glues were applied in 250 µm thickness onto the surface
of 7 different wood species: oak, black locust, ash, beech, birch, Scots pine and spruce. The wood were marked in 10 to 10 different locations before the measurements (before the adhesive coat) so the color measurement was always done in the same place. The color measurement was performed prior to adhesive application, 24 hours after application, when the adhesive layer is fully cured. The difference of the color (between the color of the wood and the changed color of the wood) was calculated with the average of measurements on 10 pre-designated places (Fig. 1).
Fig. 1. Discolouration of the wood surfuces
RESULTS AND DISCUSSION
There were differences between adhesive types and wood species too. Each color coordinates has different effect on the change of the color, so the three color coordinates has different contribution to the total color change.
The coordinate a* (red component) majority moved towards the positive range in case of the broadleaved species (Fig. 2), while in case of the pines a slight negative shift was typical. The change is usually below 1, but in case of the ash and the black locust with some types of glue, this was more than 2. The differences between the glue types the most clearly pronounced in the case of ash and black locust. In case of the black locust one of the D3 type glue (No.4) caused the far greater change than others. The other, No.3 D3 type adhesive has only a third effect as the No.4, despite the fact that the effect is more than double than the D2 type adhesives produced. In case of the ash the adhesives No.1 and No.2 give of twice as big the values as D3 types, but the other two D2 type is omitted from them. There isn’t a clear difference between the two types of glue in the change of a*, the changes are determined by the individual reactions between the different woods and adhesives.
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Robinia Oak Ash Beech Birch Spruce Scots
pine
Fig. 2. Redness change of different wood surfaces
The b* coordinate in almost every case shifted towards the yellow color (Fig. 3). The values of the changes are the majority vary between 2 and 4. So the yellow color changes accordingly more significant than the red color change. In the b* changes the differences between adhesive water resistance level are also shown. In case of all species the D3 type adhesives caused major change. This is particularly important in case of the lack locust, where adhesive products classified in lower water resistance level, caused differences in b* color coordinates is a fragment than belong to the higher one. In case of the other species these differences are much smaller. Between the changes caused by the D2 type adhesives minor differences can be observed as two of them (No.2 and No.5) caused lower changes within each tree species than the others. There is no clear difference between the D3 type glues, with the exception of black locust where the adhesive No.4 cause
Robinia Oak Ash Beech Birch Spruce Scots pine
Fig. 3. Yellowness change of different wood surfaces
The change of L* in pines showed a clear shift towards the dark shades (Fig.
4). The role of the two adhesive types (D2 and D3) in the change of the light factors and varies in species, you can not formulate a general effect. In case of the black locust, the effect of the D3 type glues are not increase as much as in case of the other two color components. However, the adhesive No.4 has greater changes than the others. The effect of D3 type adhesives is stronger. The effect is greater in case of the oak and lower in case of the spruce. At the ash, especially at the birch and the beech, the D2 type glues caused higher L* values (changes). At Scots pine it is interesting that the L*
coordinate – though only slightly, but – shifted to positive and negative directions in case of the same glue type.
-5
Robinia Oak Ash Beech Birch Spruce Scots
pine
Fig. 4. Lightness change of different wood surfaces
24 hours after coating, the total color change (E*) fall in different detectable ranges to the human eye (Fig. 5). In case of the pines most of the adhesives cause barely detectable (0.5-1.5) color change. In case of spruce the effect of the D3 type pass in the visible range (1.5 to 3), while in case of the Scots pine only the glue No.3 can do it. In case of the ash and birch the
E* values are in the noticeable range. The No.1 and No.2 of D2 type adhesives caused slightly larger changes on these two species than the D3 types. Tin case of the oak are even more significant color changes, as the values are in the visible (3-6) range and the two D3 glue approach the upper limit. In case of the black locust clearly the D3 type adhesives cause significant changes, as in the other color components were measured. The No.4 caused the highest discoloration. Until the color changes of the D2 type glues are in the noticeable (No.5, No.6) and well visible (No.1, No.2) range, while the color changes (E*) of the D3 types are over the range of high difference (6-12).
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0 5 10 15 20 25 30
Robinia Oak Ash Beech Birch Spruce Scots
pine
DE*
1 2 3 4 5 6
Fig. 5. Total color change of different wood surfaces
Looking at the change of color components, can be seen that these determine in different percentage the ΔE* in different species (Fig. 6). Overall, the red hue has the smallest effect, followed by the light changes. Mainly the change of the yellow component determined the amount of the color stimulation difference in case of most species and glues. This especially in case of the D3 type glues appears strong, with the exception of the oak, which was more determined by the darkening. In case of the D2 type adhesives there isn’t a color coordinate, which determine the overall color change independent from the species. Two of the four D2 type adhesive (No.2, No.5.) have similar proportion of the color components. In case of these two adhesives, at most of the broadleaved species the changes of the lightness are usually dominant. The other two D2 type adhesive are similar to the D3 type, as usually the yellow component has the highest proportion, but not so high extent.
0%
20%
40%
60%
80%
100%
Robinia Oak Ash Beech Birch Spruce Scots pine Robinia Oak Ash Beech Birch Spruce Scots pine Robinia Oak Ash Beech Birch Spruce Scots pine Robinia Oak Ash Beech Birch Spruce Scots pine Robinia Oak Ash Beech Birch Spruce Scots pine Robinia Oak Ash Beech Birch Spruce Scots pine
1 2 3 4 5 6
L*
b*
a*
Fig. 6. The rate of the color components in total color change
CONCLUSIONS
There are significant differences between total color changes of surface of the selected seven different species 24 hours after the adhesive material application. In case of the pine all the three color coordinates have the smallest change. So regardless of the type of the glue the color change is not considerable according to the other species. Until there isn’t any difference in the changes of the a* between the water resistance types of the adhesives, while in the changes of the b* it is clear that the D3 type of glues cause greater change. In case of the changes of the L* coordinate depend on the species which type of adhesive caused the greater change. The more powerful effects of the D3 type glues can be observed in the change of E*.
This was the strongest at the black locust, which has a special material (dihydro-robinetin) and it causes the color change by reacting with one component of the D3 type adhesives.
Overall, the adhesives cause changes in the surface color of the wood. This change depends on the type of the adhesive and the species. In case of species which rich in extract components, the reaction of the components of the adhesives used to determining the degree of water resistance with wood surface can be more powerful. There can be difference between the surface
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reactions of the same types of adhesives depending on components of the adhesive.
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CHANG, H.T. AND CHANG, S.T. (2001) Correlation between softwood discoloration induced by accelerated lightfastness testing and indoor exposure. Polym. Degrad. Stab. 72:361-365
CHANG, T.C., CHANG, H.T., WU, C.L., CHANG, S.T. (2010) Influences of extractives on the photodegradation of wood. Polymer Degradation and Stability 95: 516–521
HANSMANN, C., DEKA, M., WIMMER, R., GINDL, W. (2006) Artificial weathering of wood surfaces modified by melamine formaldehyde resins.
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SHARRATT,V., HILL,C.A.S., KINT,D.P.R. (2009) A study of early colour change due to simulated accelerated sunlight exposure in Scots pine (Pinus sylvestrris). Polym. Degrad. Stab. 94:1589-1594
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